The present invention relates in general to fuel burners and in particular to a new and useful combined burner and NO.sub.x port for burning fossil fuels.
Low NO.sub.x pulverized coal-fired burners, such as the burner disclosed by U.S. Pat. No. 5,199,355, rely on principles of air and fuel staging to reduce emissions of NO.sub.x. The effectiveness of these measures depends upon the design of the burners and the furnace to which they are applied, amongst other factors. In order to further reduce NO.sub.x emissions, NO.sub.x ports (over fire air ports, air staging ports) are employed in order to remove a portion of the air from the burners for introduction downstream in the combustion process.
In typical wall fired utility boiler applications, the burners are arranged in multiple elevations on the front and/or rear wall of the lower furnace. Low NO.sub.x burners are installed at these locations for new boilers, or retrofitted to existing boilers. For a given application, the actual NO.sub.x emissions from these burners vary across the height of the burner zone due to the changing thermal environment. The bottom elevation of the burners resides in the coolest portion of the furnace and produces the lowest NO.sub.x emissions. The top elevation of the burners produces the highest NO.sub.x since temperatures in the furnace at that location are reaching a maximum. This contributes to the formation of thermal NO.sub.x.
In addition, the upward flow of gases from the lower burner elevations impinge on the flames of the upper burners, accelerating the mixing of air and fuel, which contributes to fuel NO.sub.x. These effects are documented in numerous tests of boilers, which show that removing the top row of burners from service reduces NO.sub.x, while removing the bottom row of burners from service increases NO.sub.x (compared to all burners in service). NO.sub.x ports have become necessary to achieve NO.sub.x emission objectives. NO.sub.x ports are normally positioned above the top burner elevation; and the effectiveness of the NO.sub.x ports is a function of how much air is diverted from the burners to the ports, and the distance from the burners to the ports. However, in many existing boilers it is difficult to find a suitable location above the burners to locate the ports. The height of the furnace or arrangement of the heating surface or the auxiliary equipment prevents the addition of ports above the burners.